A plug and a socket are conductive when supplied with single-frequency Alternating Current (AC) power. Once the AC power is cut off, an electrical device stops working due to a non-conductive circuit. On the one hand, a power outage may damage components inside the electrical device; on the other hand, the power outage affects normal running of the electrical device. For an electrical device used on an important occasion, a shutdown causes unnecessary economic losses and may, if serious, threaten safety of a user. Therefore, it is needed to provide a power plug and socket to ensure normal running of an electrical device after AC power is cut off.